The invention relates to soap stabilized emulsions and to the methods of manufacture of such emulsions, and in particular to silicone emulsions useful in personal care products.
Traditionally, soap in the form of bars or flakes has been used as the preferred cleaning agent in personal care. In recent years, personal cleaning agents in the form of gels have become increasingly popular. These gels are sometimes referred to as shower gels. Such shower gels are extremely popular with consumers.
These shower gels commonly contain:
Detergents or surfactants which can be synthetic or natural soap.
Humectants such as glycerine and propylene glycol.
Chelating agents such as EDTA.
Buffers such as citrate or citric acid.
Pearlisers such as EGMS and mica.
Perfumes and Colours.
Thickeners such as hydroxy ethyl cellulose.
Water.
It is believed that the addition of silicones (commonly polydimethylsiloxane) to these gel formulations adds beneficiary sensory effects to the skin which may be observed during or after the use of the gel.
The proportions of each ingredient are selected to provide good clearing ability and also to result in a product with a texture pleasant to the consumer. Shower gels can be classified according to the type of surfactants present e.g. all-synthetic surfactant, blend of soap with synthetic surfactant and all natural soap with no synthetic surfactant.
A xe2x80x9cSoapxe2x80x9d as herein defined is composed of one or more carboxylates of long chain fatty acids in combination with one or more cations.
Consumer tests in markets where shower gels are most popular have shown that consumers can detect the difference between a shower gel containing synthetic surfactants and one containing all natural soap, and clearly prefer the 100% soap based gel.
All-synthetic surfactant based products are generally more stable and more easily prepared than soap based products. A number of silicone based compounds give suitably stable emulsions in conjunction with these all-synthetic surfactant products. While these emulsions are relatively easy to manufacture, and have good stability, they suffer from the drawback that consumers can detect a reduction in agreeable skin-feel of the product when compared with a 100% soap based gel. All-synthetic surfactant based products are also less attractive from a marketing point of view as they are not perceived by consumers as having the desirable property of being derived from natural sources. Shower gels containing a 100% soap (that is to say substantially exclusively soap as the surfactant), typically contain around 25% soap in total. As mentioned above, 100% soap based gels are preferable from a marketing point of view, because of their good skin-feel properties and their being perceived as more xe2x80x9cnaturalxe2x80x9d than synthetic surfactant based gels.
When manufacturing 100% soap based gels, the key raw materials are the long chain fatty acids which form the soaps. These tend to be low melting solids or waxes which are difficult to process. Heating may be required when preparing the soap.
An even greater problem of the 100% soap based shower gels comprised of a silicone oil-in water emulsion is that such a combination of the soap based shower gel and the silicone oil is not stable. Currently no stable silicone emulsion with a 100% soap is known. Typically, the product begins to separate and exhibit distinct layers ie. a clear, lower water layer and an opaque layer containing components which are not water soluble or water miscible. This can happen quite rapidly, in some cases overnight. Such separated products are generally unacceptable to consumers.
Some silicone compounds provide limited stability for emulsions based on a blend of soaps and synthetic surfactants, with typically more than 5% synthetic surfactant being required. Shower gels which contain a synthetic surfactant/soap blend typically contain around 5 to 15% soap therein. Two types of synthetic surfactant can be used in combination with the soap, namely amphoteric surfactants (for example betaines) or non-ionic surfactants (for example CDEA (coconut diethanolamines)). These surfactants are generally added at a level of 5% into the synthetic/soap surfactant blend. Generally silicone oil-in-water emulsions prepared using synthetic surfactants are stable in synthetic surfactant and soap/synthetic surfactant blend (generally provided that the synthetic surfactant level in the blend is greater than or equal to 5%) based shower gels, but such emulsions are not stable in soap surfactant shower gels.
The present invention seeks to overcome at least some of the disadvantages of the prior art or at least provide a commercial alternative thereto.
According to a first aspect the present invention relates to a silicone oil emulsion stabilized with soap, wherein the soap includes:
one or more carboxylates of a fatty acid having from 8 to 18 carbon atoms; and
a cation of a base, the soap being formed in situ from the fatty acid and the base during formation of the emulsion.
Preferably, the one or more carboxylates is a mixture having a similar distribution to that found in vegetable oils. More preferably, the distribution is that found in coconut oil or palm kernel oil. Most preferably, the mixture is derived from coconut oil. Table 1 shows typical compositions of Coconut and Palm oil which are suitable for use in the present invention.
It will be appreciated by those skilled in the art that compositions of coconut and palm kernel oil vary somewhat depending on the geographical origin, soil composition etc. The average compositions are summarised for the purposes of information only in the Table. The average compositions must be understood such that they may contain other fatty acids which have more or less carbon atoms than those described in the table. The compositions described in the table do not show any particular oil available from a single supplier, but rather give representative compositions. If it is desired to know the exact composition of the oil, this must be examined prior to production of the silicone emulsion.
These oils are available commercially, or may be obtained as precursors which may be converted into the desired fatty acids by chemical treatment such as hardening, cracking, saponification etc depending on the necessity thereof.
The base is one which may be reacted with the above fatty acids to form a soap. Preferably, the base is a trialkanolamine species, most preferably [HN(CH3CH2OH)3]. However, mineral caustics, such as sodium hydroxide and potassium hydroxide may also be used.
Preferably the silicone oil is a xe2x80x9csilicone fluidxe2x80x9d, and more preferably a polydiorganosiloxane. Such a polydiorganosiloxane is a linear polymer where the organo radical may be hydrogen, an alkyl group such as methyl, ethyl, propyl, butyl, an aryl group such as phenyl, an alkenyl group such as vinyl and the like. Preferably, the polymer terminates with xe2x80x94Si(CH3)3 (xe2x80x9ctrimethyl endcappedxe2x80x9d) or xe2x80x94Si(CH3)2OH (xe2x80x9chydroxy endcappedxe2x80x9d) moieties. Preferably, the silicone oil used has a viscosity below 1,000,000 cps and more preferably below 100,000 cps. The oil may contain other functional groups, such as carboxy, halo etc. or any combination thereof.
The silicone oils of this invention are well known to those skilled in the art, with the most highly preferred compound being trimethylsilyl terminated polymethylsiloxane having the viscosity range described above.
As discussed below, the mean particle size of the emulsion is critical to provide a stable emulsion for use in a 100% soap based shower gel. Preferably the emulsion of the present invention is a xe2x80x9csubmicro-emulsionxe2x80x9d with a mean particle size of less than 1.0 micrometers. More preferably the mean particle size in less than 1.0 micrometers and most preferably it is between 0.25 and 0.4 micrometers. Preferably at least d(90%) of the particles in the emulsion are less than 0.6 micrometers in size.
The present applicants have found that although the use of a single long chain fatty acid has been able to provide a stable silicone emulsion, these have not been suitable in shower gels. A blend of long chain fatty acids, wherein the carbon backbone varies from C8 to C18 has been found to produce a more stable emulsion.
Further, it has also been found that a significant and hitherto unexpected difference in the stability of the emulsion results from varying the cations of the soap present. The use of a trialkanolammonium cation based soap has been found to produce a surprisingly stable emulsion.
In the present invention, it is important to form the soap by mixing the fatty acids and the base in the emulsification process, that is, the soap must be formed in-situ. Preformed soap was found not to provide such stable emulsions.
According to a second aspect the present invention relates to a method of preparing a silicone oil-in-water emulsion including;
forming a first mixture including a silicone oil, a base, and initial water;
combining the first mixture with one or more fatty acids having from 8 carbon atoms to 18 carbon atoms;
emulsifying the resultant combination; and
wherein a soap is formed in situ in the process of emulsification.
If the one or more fatty acids are a wax or a solid, then preferably it is liquefied prior to combining with the first mixture. Preferably the liquification occurs by heating.
The silicone oil may be present in the oil-in-water emulsion in the broad range of from a few percent by weight or less to around 70%. The amount is not critical, as a skilled addressee may be able to determine this for the purpose of the present invention. Preferably in the case of the shower gel, the silicone oil is added in an amount such that it totals from 30% to 60% w/w of the resultant emulsion. More preferably the silicone oil totals around 50-60% w/w of the resultant emulsion. Preferably the base is added in such an amount that it totals at least 1 mole equivalent with respect to the carboxylic groups of the one or more fatty acids present in the mixture. Preferably, the initial water is added in such an amount that it totals from 3% to 10% w/w of the resultant emulsion. More preferably the initial water totals around 3.3% w/w of the resultant emulsion.
Preferably the emulsification is by mechanical agitation means. The agitation means include, but are not limited to, a homomixer, emulsifier, homogenizer or colloidal mill. More preferably, the emulsification is by high shear means. An example is a trishaft mixer with two high speed disperser shafts and one anchor scraper, known in the art as a xe2x80x9cChange Canxe2x80x9d, xe2x80x9cTurelloxe2x80x9d or xe2x80x9cCombixe2x80x9d mixer.
The soap stabilized oil-in-water emulsion can further include addition of additional water. This additional water, for example, dilution water, can be added during emulsification or subsequent to emulsification to provide the desired concentration of effective component.
Optionally the mixture can include a biocide.
Preferably if additional water and/or biocide is added, this is done so under vacuum to reduce foaming.
The first mixture is prepared by mixing the silicone oil, the base and the water, followed by addition of a second mixture of one or more fatty acids having from 8 carbon atoms to 18 carbon atoms. The resultant mixture is mechanically sheared to produce a soap from the base and the fatty acid(s) in situ with concurrent emulsification of the mixture. Additional water may be added during and/or subsequent to the emulsifying process to produce a final stabile emulsion.
The addition of initial water is important in terms of achieving the desired particle size. A value for initial water is chosen such that two criteria are satisfied: i) a thick phase emulsion forms and ii) the desired particle size is reached. The first criteria sets the minimum value, as if it is too low there is not enough water to form a continuous water phase and the second criteria sets the maximum value since if too much initial water is present the surfactants are too dilute to achieve particle size reduction. Once the desired particle size is achieved the dilution water (and minors such as biocide) can be added. In general, the steps for forming the sub-micron soap emulsions of the present invention are:
1. Mix triethanolamine (base), silicone fluid/polymer, and initial water in a Change Can until uniform
2. Add melted fatty acid to form soap under medium shear
3. Increase shear to reduce particle size
Once the desired particle size is achieved, any dilution water and minor ingredients are added under vacuum to prevent aeration.
The stable silicone oil-in-water emulsion of the present invention is accomplished by forming the soaps in situ by adding the fatty acid(s) and the base individually, thereby achieving the emulsion of very small particle sizes.
Whilst any silicone oil is suitable for use in the present invention, polydiorganosiloxanes are generally preferred. The most suitable poly siloxanes are discussed above.
A blend of long chain fatty acids, wherein the carbon backbone varies from C8 to C18, has been found to produce a more stable emulsion than a single long chain fatty acid or a synthetic surfactant.
An explanation as to why a blend of fatty acids have better stability than single chain length soaps is given below, although it is to be understood that such an explanation in no way limits the scope of the monopoly sought. Soaps obtained from different fatty acids will position themselves differently at the particle water interface. Varying the length of the fatty acid chain will cause changes to the acidity and the hydrophobicity of the soap molecule, which in turn is thought to vary the way different soap molecules provide particle stabilisation.
Whilst the mechanism of vesicle formation (ie. a droplet of silicone oil with fatty acids surrounding it, charge outwards) has not been thoroughly investigated, it will be appreciated that the nature of such a vesicle will be different if a mixture of different fatty acids is used rather than a single compound where all the molecules are of the same length.
Surprisingly, it has also been found that a significant and hitherto unexpected difference in the stability of the emulsion results from varying the cations present. Soaps can be formed from many different cations, for example, K+, Na+ or protonated amines such as triethanolamine, aminomethylpropanol etc. The tricthanolammonium cation has been found to produce very stable emulsions when used in the present invention. This has been observed to be the case both for the silicone oil-in-water emulsions per se and also when these are incorporated into complete shower gel products.
It is postulated that the larger size of this cation and the shielding of the charge on the central nitrogen atom by the ethanol chains results in an emulsion where there is far less mobility of charge.
It is also possible that such mixtures of fatty acids alone or in conjunction with the use of a triethanolamine may have application in other areas of personal care, such as more traditional solid soap bars, or in unrelated industries such as the textile and plastics industries.